The present invention relates to an exhaust system for an outboard motor with enhanced engine performance by preventing exhaust interference from occurring between a plurality of cylinders.
In a multi-cylinder four-stroke engine which is mounted in a vehicle, such as an automobile or a motorcycle, a plurality of exhaust pipes coming from each cylinder, each having a different respective exhaust timing, are joined together to make a group exhaust. By eliminating both positive pressure waves and negative pressure waves, which are generated by the pulsation of the exhaust gases, the charging efficiency of the intake fuel-air mixture and the engine performance, such as output characteristics and torque characteristics, are improved. In four cylinder engines, for example, four exhaust pipes from each cylinder are joined into two, and subsequently into one; thereby forming a 4-2-1 type group exhaust system that is often used. In addition, it is desirable that the differences in lengths between the exhaust pipes extending from each cylinder are small.
However, in an ordinary four-stroke engine for an outboard motor, the engine is mounted vertically so that the crankshaft is oriented in a vertical direction, as shown in FIG. 11. Exhaust passages 101, which extend vertically, are provided along the side of the cylinder block 100. In a four-cylinder engine, for example, four exhaust ports (not shown) from four cylinders 102A-102D communicate with the exhaust passage 101. Exhaust gases discharged from each cylinder 102A-102D are collected inside the exhaust passage 101, flow downward and are subsequently discharged into water.
In this type of outboard motor, group exhaust does not occur. All exhaust gases discharged by each cylinder 102A-102D are collected inside one exhaust passage 101. As a result, it has the disadvantage that exhaust interference occurs among those cylinders which have the same exhaust timing, which results in decreased output or worsened torque characteristics.
In order to prevent such exhaust interference from occurring, and to apply the 4-2-1 group exhaust system discussed above, one easy method involves providing a separate exhaust manifold to the cylinder block, as disclosed in Japanese Laid-Open Patent Publication No. Hei-9-49425.
However, if, as described above, a separate exhaust manifold is mounted to the cylinder block, the number of components will increase, which will cause productivity and assembling efficiency to worsen. Additionally, in the case of an outboard motor, the engine is covered by an engine cover, and air inside the engine cover is used as intake air for the engine. However, the temperature within the engine cover is increased by the heat emitted from the exhaust manifold. Due to this, the density of the intake air is decreased, which results in the possibility of decreased performance or decreased durability of components. To prevent this, it is necessary to form a water jacket around the exhaust manifold, and thereby the structure of the manifold becomes complex and leads to larger manufacturing costs. Further, the possibility of problems, such as water leaks, increase accordingly.
The four-stroke engine for an outboard motor of the present invention was invented to solve the problems described above. Having a simple structure, good productivity and design characteristics, the exhaust system of the present invention makes group exhaust possible, prevents exhaust interference from occurring, and increases the cooling efficiency of the exhaust passages.
In order to solve the problems described above, the present invention provides an exhaust system for an outboard motor, which, as described in claim 1, is equipped with a four-stroke engine having a cylinder block comprising a plurality of cylinders which are arranged vertically so that the crankshaft is vertically oriented. The exhaust system includes a plurality of separate exhaust passages that extend vertically and are formed integrally with the cylinder block. Each of the exhaust ports from each cylinder which have different respective exhaust timings communicates with one of the plurality of exhaust passages.
By constructing the exhaust system for an outboard motor as described above, as a plurality of independent exhaust passages are integrally formed along with the cylinder block, it becomes possible to discharge exhaust gases in groups, while making the exhaust passages of a simple structure without increasing the number of components for the engine. This results in enhanced engine performance. Further, as the exhaust passages are easily formed along with the cylinder block by die-casting, the manufacturing efficiencies of the engine (cylinder block) and the exhaust system are extremely good.
Also, in the present invention relating to an exhaust system for an outboard motor the exhaust port from the cylinder disposed in the lowest position out of the plurality of the cylinders communicates with the most distantly disposed exhaust passage out of the plurality of exhaust passages. With this arrangement, it becomes possible to form the exhaust port from the cylinder disposed at the lowest position longer than the exhaust port from the cylinder disposed at higher positions. Thus, the differences in lengths of the exhaust passages, including the exhaust port, are decreased. As a result, exhaust interference is more effectively prevented from occurring.
Further, in the present invention relating to an exhaust system for an outboard motor the plurality of exhaust passages are joined together at a point which is farther downstream than the point where the exhaust port from the lowest cylinder communicates with one of the exhaust passages. With this arrangement, all of the cylinder exhaust ports communicate with one of the exhaust passages, and then the exhaust passages are joined together. Therefore, effective group exhaust becomes possible. Further, the exhaust passage that is formed from components below the cylinder block is made into one simple structure.
Still further, in the present invention relating to an exhaust system for an outboard motor the plurality of exhaust passages are arranged to be disposed in a line in the width direction of the outboard motor. With this arrangement, by only changing the length of each exhaust port, it becomes possible to select a combination of groups of exhaust passages, as well as to make a simply designed exhaust system.
Moreover, in the present invention relating to an exhaust system for an outboard motor the plurality of exhaust passages are arranged to be disposed in a line in the front to rear direction of the outboard motor. By this arrangement, because a plurality of exhaust passages do not protrude over the side face of the cylinder block, it becomes possible to provide a plurality of exhaust passages to make effective use of space, as well as to make a simple exhaust system design.
Furthermore, in the present invention relating to an exhaust system for an outboard motor a water jacket is formed within the bulkhead that partitions the plurality of exhaust passages. By including this water jacket, it becomes possible to increase the cooling efficiency within the exhaust passages.
Still further, in the present invention relating to an exhaust system for an outboard motor the water jacket formed within the bulkhead communicates with a water jacket that is provided around the exhaust passages. By this arrangement, the ability of cooling water to circulate within the water jacket in the bulkhead improves. The cooling efficiency of the exhaust passages is also greatly improved.
The present invention also provides an exhaust system for an outboard motor is equipped with a four-stroke engine having a cylinder block that includes a plurality of cylinders and a plurality of separate exhaust passages. The cylinders are arranged vertically so that a crankshaft is vertically oriented and the cylinders have different respective exhaust timings. Each cylinder has an exhaust port. Each exhaust passage extends in the vertical direction and is formed integrally with the cylinder block. Each exhaust port from each cylinder communicates with at least one of the plurality of exhaust passages.
In one embodiment of the present invention relating to an exhaust system for an outboard motor the plurality of exhaust passages are comprised of an inner exhaust passage and an outer exhaust passage which are arranged in a line in the width direction of the outboard motor. The exhaust port from the cylinder disposed in the lowest position out of the plurality of cylinders communicates with the outer exhaust passage.
In a second embodiment of the present invention relating to an exhaust system for an outboard motor the plurality of exhaust passages are comprised of a front exhaust passage and a rear exhaust passage which are arranged in a line in the front to rear direction of the outboard motor. The structure further includes a joint passage, such that the exhaust port from the cylinder disposed in the lowest position out of the plurality of cylinders communicates with the front exhaust passage through the joint passage.
Also, in the present invention relating to an exhaust system for an outboard motor the plurality of exhaust passages are joined to each other at a point below the exhaust port connection point of the lowest cylinder.
Further, in the present invention relating to an exhaust system for an outboard motor a water jacket is formed within a bulkhead that partitions the plurality of exhaust passages. The water jacket formed within the bulkhead communicates with a water jacket that is provided around the plurality of exhaust passages.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.
The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several embodiments of the invention and together with the description, serve to explain the principles of the invention.